Only the coefficients will change in value.
here, just add the coefficients, of the same variable, taking into consideration the sign also.
the coefficients are -1 and +12; when we add those two , we get +11
Let me give you another example.
we have two variables here x and y
first take the coefficients of x; -5 and +12. Adding them will give you +7
Noy coefficients of y; +8 and -19. Adding them will give you -11
so the resulting expression will be 7x-11y
Hope this helps.